In the British legal tradition, the Royal Prerogative of Mercy allows the monarch to grant pardons to convicts.

In practice, this power is delegated to the Home Office or the Northern Ireland Office.

The power was originally used to permit the monarch to withdraw death sentences, but it is now used to change any sentence or penalty.

With the news that 187 letters were sent to OTRs telling them they were no longer being pursued, questions are now being asked as to what other deals were done behind closed doors.

Attention is turning towards the issue of the Royal Pardons, highlighted by the case of republican Gerry McGeough in 2010 and a decade earlier in the case of four IRA men.

The DUP last night refused to be drawn on why the party has not hit out at their use previously, but party leader Peter Robinson said after his meeting with Secretary of State Theresa Villiers that Royal Pardons were part of the crisis.

McGeough revealed in April 2010 that dozens of OTRs had been told they were free to return to Northern Ireland.

Just weeks previously former Secretary of State Shaun Woodward had denied any secret deal, dismissing the claims as "complete nonsense".

The document produced by McGeough showed "incontrovertible evidence that (Royal) pardons were granted", lawyers for him said in 2010 when he stood trial for IRA membership and the attempted murder of a UDR man.

Issued in the name of Her Majesty The Queen, the letter stated an unnamed republican had been pardoned and remitted from a sentence of imprisonment imposed on him.

The individual issued with the pardon presented by McGeough escaped from Crumlin Road jail in Belfast in 1981.

The document dated December 24, 2000 read: "This is to certify that Her Majesty The Queen has been pleased to extend Her Grace and Mercy unto (name removed) and to pardon and remit to him the unserved portion of all the fixed term sentences of imprisonment imposed on him at Belfast on June 12, 1981.

"He is therefore deemed to have served all the said sentences in full and is, accordingly, no longer unlawfully at large."

In 2012 McGeough lost his legal challenge against his subsequent conviction at the Court of Appeal. Senior judges dismissed his bid to be granted a Royal Prerogative of Mercy because he had not served at least two years in a British or Irish prison.

The use of Royal Pardons had been widely reported more than a decade earlier.

In 2000, former IRA prisoners who escaped from prison in Belfast were granted special dispensation by the Queen to return to Northern Ireland.

The Royal Prerogative of Mercy given to Angelo Fusco, Robert Campbell, Paul Patrick Magee and Anthony Gerard Sloan meant they were free from any risk of prosecution.

The four were among a group of eight who broke out of Crumlin Road jail in June 1981.

The move endorsed an announcement made by then Secretary of State Peter Mandelson earlier in 2000 that convicted terrorists living outside Northern Ireland would not be pursued.

They were convicted in their absence for their part in the murder of Captain Herbert Richard Westmacott, the highest ranking member of the SAS to be killed in the Troubles.

He was shot dead by an IRA unit on the Antrim Road in the north of the city in May 1980.

The men were all later recaptured in the Republic, where they were imprisoned for eight years.

Because they had served less than two years of their original sentence the IRA men did not qualify for the Good Friday Agreement's accelerated early release scheme, but then received the Royal Pardons.

Solicitor Aiden Carlin confirmed his client McGeough will raise the revelations in the Downey case with the Criminal Cases Review Commission.